Thanks for the reply 1077. I'll take a water sample to my local shop.
Its a couple of hours later now and the Tetras have returned to their normal position, but it gave me quite a fright to see them all acting so different to normal.
What I can tell you now is that I have a 240 litre, heavily planted, community tank with 20 Neon Tetras, 2 Koi Angelfish, 4 Scarlet Barbs, 2 Pearl Gouramis, 4 (I think) Blue Gouramis, 1 Kissing Gourami, 3 Bumblebee Gobis, 1 Rummy Nose Tetra, Lost the other 3
2 dunno what arrow like Tetras and 6 dunno what Shrimp.
I feed them 3 times a day, but no matter how much I give them, hardly a scrap ever makes it to the bottom of the tank for the shrimp, who try their best to swim to the top to feed.
Made my first big error by adding 2 Convict Cichlids a few weeks back and immediately noticing a complete change in every fish's behaviour - Fear, I say it was! (Took 'em back to the shop)